This invention relates to a novel spacer construction for an electrodeionization apparatus. More specifically, this invention relates to a spacer construction adapted to retain ion exchange resin beads within an ion depleting compartment while promoting even liquid flow within the compartment and preventing plugging of liquid inlets to the compartment by the beads.
The purification of a liquid by reducing the concentration of ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of specific ions or molecules from a liquid mixture. The most well known processes include electrodialysis, liquid chromatography, membrane filtration and ion exchange. A lesser known methodology is electrodeionization, occasionally mistermed filled cell electrodialysis. Although electrodeionization has the potential to be quite effective in removing ions from liquid, it has never been developed to the degree that it is competitive either structurally or operationally with the better known separation techniques. This is due primarily to the inconsistencies of structural design incurred by the presently known modes of use. This lack of structural design precision and nonpredictability of results have reduced the use of electrodeionization to the point where it is relatively unknown even to practitioners skilled in separation methodologies.
The first apparatus and method for treating liquids by electrodeionization was described by Kollsman in U.S. Pat. Nos. 2,689,826 and 2,815,320. The first of these patents describes an apparatus and process for the removal of ions within a liquid mixture in a depleting chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentration chamber under the influence of an electrical potential which causes the preselected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transferred ions and carries them in concentrated form. The second of these patents describes the use of macroporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ion exchange resin acts as a path for ion transfer and also serves as an increased conductivity bridge between the membranes for the movement of ions. These patents represent the primary structural framework and theory of electrodeionization as a technique. The term electrdeionization refers to the process wherein an ion exchange material is positioned between the anionic and cationic diaphragms. The term electrodialysis relates to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Despite the fact that the Kollsman technique has been available for over 25 years, this technology has not been developed even to the point of practical use. This is due in no small part to the lack of structural designs and the unavailability of operational mode parameters which afford reliable operation of the electrodeionization apparatus. Illustrative of prior art attempts to use the combination of electrodialysis and ion exchange materials to resins to purify saline from brackish water are described in U.S. Pat. Nos. 2,794,777; 2,796,395; 2,947,688; 3,384,568 and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517 and 4,284,492.
Despite the contributions of the prior art, no reliable electrodeionization apparatus has been produced. The typical device fouling and scaling problems of electrodeionization remain unalleviated. Presently described electrodeionization apparatus remain unsuitable for desalination or for the production of high purity water. Hard waters, silica-containing waters and highly saline brackish waters, and waters containing colloidal particles and foulants still represent liquids that cannot be consistently and reliably purified by presently known electrodeionization apparatus and modes of operation. Extensive maintenance and cleaning of these apparatus remain necessary, the quality and volume of the purified liquids remain erratic and the ability to produce at least 1 meg-ohm centimeter quality water consistently and in sufficient volume remain unachieved.
Furthermore, the use of resin beads can lead to channeling of liquid through the depleting compartment rather than even flow across the width of the compartment. Liquid channeling can cause undesirable incomplete treatment of the ion containing liquid since it contacts only a small portion of the resin beads which rapidly lose their capacity to conduct ions. In addition, the resin beads can migrate to the depleting compartment inlets and outlets and block them.
Accordingly, it would be desirable to provide an electrodeionization apparatus capable of producing a high purity deionized water over long periods of time without the need for shutting down the apparatus either because of reduction of ion exchange resin performance or because of particle or scale build-up. Furthermore, it would be desirable to provide such an apparatus which promotes even flow through the depleting compartments and which can be operated efficiently with low energy requirements and high liquid throughput.